Advice wanted - Training/Diet/How to approach...

Hey guys, never posted in the training section before. Thought I'd pick all of your brains, I've got quite alot for you, so bear with me and please help me out if you can.

I'm 19 years old, I used to box when I was younger. I was a fast southpaw and my reflexes were pretty well tuned. I built my style around this. As I got into my teenage years I became a bit of a muppet, when I was angry over something I'd punch a wall, I did that one too many times and I messed my hands up pretty bad. (I don't need to be told how stupid I was) so now, I get random pains through the bones in each of my hands (my left especially) I can't even punch an empty coke bottle without getting a small twinge of pain.

4 years ago : (Making me 15)
I had competed in 9 amatuer fights, I'd won all 9, 3 by stoppage (amatuer stoppages, I could never hit hard enough to destroy someone, but refs are pretty quick to jump in, as you know.) In my 10th, I broke my left hand and was pulled out by my corner. I never boxed in a competition again.

A few months back, I realised how unfit I am. I've played football (soccer) for a few years but I can pretty much play my position well enough without needing stamina so I never bothered to build it. I've decided to box again.
I went into a ring for the first time in years a few months ago, with a friend of mine who's a boxing fan, yet has never boxed properly. I was shocked, I've lost my speed, timing, reactions, almost the lot. Please tell me I can get these back.

I would like your advice please, on the following...

How to build my fitness up from basically nothing. (Before going back to the boxing gym.(My local fitness gym has pads, a heavy bag, a ring, skipping ropes etc)

I'd like to gain a little weight - 19, 6 foot and 144 pounds doesn't seem right for me.

Diet advice, meals, how many, consisting of what, etc.

Any other helpful tips in terms of protecting my hands etc.

Thanks very much.